Quantum dot solar concentrators: Electrical conversion efficiencies and comparative concentrating factors of fabricated devices

A novel, non-tracking concentrator is described, which uses nano-scale quantum dot technology to render the concept of a fluorescent dye solar concentrator (FSC) a practical proposition. The quantum dot solar concentrator (QDSC) comprises quantum dots (QDs) seeded in materials such as plastics and glasses that are suitable for incorporation into building façades. Photovoltaic (PV) cells attached to the edges convert direct and diffuse solar energy collected into electricity for use in the building. Small scale QDSC devices were fabricated. Devices have been characterised to determine current, voltage and power readings. Electrical conversion efficiencies, fill factors and comparative concentrating factors are reported.     

Improving the optical efficiency and concentration of a single-plate quantum dot solar concentrator using near infra-red emitting quantum dots

Low luminescent quantum yields and large overlap between quantum dot (QD) emission and absorption spectra of present commercially-available visible-emitting QDs have led to low optical efficiencies for single-plate quantum dot solar concentrators (QDSCs). It is shown that using near infra-red (NIR) emitting QDs, re-absorption of QD emitted photons can be reduced greatly, thereby diminishing escape cone losses thus improving optical efficiencies and concentration ratios. Using Monte-Carlo ray-trace modelling, escape cone losses are quantified for different types of QD. A minimum 25% escape cone loss would be expected for a plate with refractive index of 1.5 containing QDs with no spectral overlap. It is shown that escape cone losses account for ∼57% of incident photons absorbed in QDSCs containing commercially-available visible-emitting QDs.     

Multi-stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs

We report GaAs-based quantum dot (QD) solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a highly stacked and well-aligned InGaAs QD structure of over 30 layers without using a strain compensation technique by the intermittent deposition of InGaAs layers. Moreover, there was no degradation in crystal quality. The external quantum efficiency of multi-stacked InGaAs QD solar cells extends the photo-absorption spectra toward a wavelength longer than the GaAs band gap, and the quantum efficiency increases as the number of stacking layers increases. The performance of the QD solar cells indicates that the novel InGaAs QDs facilitate the fabrication of highly stacked QD layers that are suitable for solar cell devices requiring thick QD layers for sufficient light absorption.     

A theoretical approach on the strain-induced dislocation effects in the quantum dot solar cells

The numbers of the quantum dot layers that can be embedded in the active region of the quantum dot intermediate band solar cells affects on the photocurrent and also can produce strain-induced dislocations in the cell. To enhance the absorption of the low energy photons in the system, the number of the quantum dot layers needs to be increased, but in this way, dislocations and defects of the cell non-radiative recombination will also increase. In this paper, the characteristics of intermediate band solar cells containing 10, 20, and 50 InAs quantum dot layers embedded in the active region of the cells have been considered and compared. There are an optimum number of quantum dot layers for significant absorption of low energy photons. Furthermore, for a cell with 10 QD layers, the current–voltage characteristics and internal quantum efficiency have been investigated for different values of minority carriers recombination lifetimes (or diffusion lengths) and electron filling factors. Electron filling factor, gives a design constraints for the size of the quantum dots and distance between the layers. The results showed that the perfect cells need to be considered from two aspects; first, from the optimum number of the quantum dot layers to control the strain-induced dislocations that produce non-radiative recombinations and reduce the photocurrent and second, the dots spacing and size that need to be justified for wavefunction penetration into barrier region that reduces the non-radiative recombinations.     

Thermal transport within quantum-dot nanostructured semiconductors

In this work, we aim at exploring the effects of the germanium quantum dot (QD) layer embedded in silicon thin films on the thermal transport property in use of the non-equilibrium molecular dynamics simulation tool. An attempt is made to distinguish and understand the effect of the QDs themselves and the effect of the wetting layer on which QDs are grown. In this study, we notice as often observed a significant increase in the thermal resistance due to heterogeneous interfaces. Moreover, it is found that a simple QD interface has a thermal resistance monotonically decreasing with increasing quantum dot density. It is probably because the QDs make the transition from one material to another smoother, alleviate the acoustic mismatch, and thus assist the energy transport. When the germanium QDs together with a germanium wetting layer is inserted into a silicon material, the involved interface thermal resistance decreases first but increases later with increasing quantum dot density. The competition between the roughness effect and the wave interference effect is employed to explain this variation trend. As far as the quantum-dot superlattice thin film is concerned, we find its effective thermal conductivity decreases monotonically with increasing quantum dot density and with decreasing film thickness. In all cases, the size of quantum dots affects little on the thermal resistance/conductivity.     

Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandem solar cells

Doping of Si quantum dots is important in the field of Si quantum dots-based solar cells. Structural, optical and electrical properties of Si QDs formed as multilayers in a SiO₂ matrix with various phosphorus (P) concentrations introduced during the sputtering process were investigated for its potential application in all-silicon quantum dot tandem solar cells. The formation of Si quantum dots was confirmed by transmission electron microscopy. The addition of phosphorus was observed to modify Si crystallization, though the phosphorus concentration was found to have little effect on quantum dot size. Secondary ion mass spectroscopy results indicate minimal phosphorus diffusion from Si QDs layers to adjacent SiO₂ layers during high-temperature annealing. Resistivity is significantly decreased by phosphorus doping. Resistivity of slightly phosphorus-doped (0.1 at% P) films is seven orders of magnitude lower than that of intrinsic films. Dark resistivity and activation energy measurements indicate the existence of an optimal phosphorus concentration. The photoluminescence intensity increases with the phosphorus concentration, indicating a tendency towards radiative recombination in the doped films. These results can provide optimal condition for future Si quantum dots-based solar cells.     

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of J_sc, V_oc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the V_oc degradation while maximizing the short circuit current density (J_sc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in J_sc and V_oc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells.     

A new approach to modelling quantum dot concentrators

Luminescent collectors have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. However, development has been limited by the performance of luminescent dyes. We have recently proposed a novel concentrator in which the dyes are replaced by quantum dots (QDs). Advantages over dyes include that the absorption threshold can be tuned by choice of dot diameter, and that the red shift between absorption and luminescence is related to the spread of dot sizes. In this paper we discuss how we have developed a self-consistent thermodynamic model for planar concentrators which allows for re-absorption by the QDs.     

Photo-stability and performance of CdSe/ZnS quantum dots in luminescent solar concentrators

The performances of luminescent solar concentrators (LSCs) made with two versions of quantum dots (QDs) with CdSe cores and ZnS shells are compared to LSCs containing the organic dye, Lumogen^® F Red 300 (LR), to assess the viability of QD LSCs. In addition to spectroscopic and light collection measurements, the photo-degradation response of the version I (vI) QD LSC is compared to the LR LSC. The measured fluorescence quantum yield of the version II (vII) QDs (57%) is about half that of LR (>90%) and twice that of the vI QDs (31%). Though the quantum yield for vII QDs is lower than LR, the vII QD LSC has nearly twice the short-circuit current of the LR LSCs or the vI QD LSCs when their respective red-peak optical densities are the same in 6.2 × 6.2 × 0.6 cm LSCs. This is a reflection of the main advantage of QDs for use in LSCs, that QDs collect considerably more sunlight than LR due to their broad absorption spectrum. Despite the fact that the QD LSCs absorbs more photons than the LR LSCs, the slow phase of the photo-degradation rate of the QD LSC is approximately five times slower than the LR LSC under nearly constant light exposure. Most surprising is the observation that the photo-degradation of the QD LSC’s absorption completely recovers during a prolonged dark cycle. In a normal day/night cycle, this will benefit the performance of the QD LSC.     

Enhanced hydrogen production over CdSe QD/ZTP composite under visible light irradiation without using co-catalyst

A series of CdSe quantum dot (QD)/zirconium titanium phosphate (ZTP) was synthesized by solvothermal method using ethylene diamine by varying Cd to Se ratio from 1:1 to 1:4 and examined as robust catalysts for hydrogen evolution under visible light irradiation without using any co-catalyst. Extensively, the structural, optical, morphological, elemental and photoresponse spectra measurement of the composite system was studied. The catalytic activity of the materials was correlated with photoluminescence spectra, band gap energy and the photosensitization effect of CdSe quantum dot. Though neat CdSe quantum dot and zirconium titanium phosphate (ZTP) exhibited photocatalytic hydrogen evolution, the composite material showed remarkable high activity. Among these, 1CdSe quantum dot/zirconium titanium phosphate (ZTP) composite showed the highest hydrogen production (905.4 μmol) within 3 h which is consistent with low photoluminescence (PL) intensity, wide band gap energy and the photosensitization effect of CdSe quantum dot.     

一步法制备高效TiO2/PbS异质结量子点太阳电池

采用一步涂层法制备TiO₂/PbS异质结且带有不同浓度PbS量子点光吸收层的太阳电池器件。测试结果表明,用浓度为200 mg/mL的PbS量子点制备的太阳电池在AM1.5模拟光照下获得的能量转换效率（PCE）为9.08%,其开路电压（V_OC）为0.570 V、短路电流（J_SC）为29.6 mA/cm²、填充因子（FF）为0.539。研究证实了一步法的可行性与可靠性。与传统的层层旋涂法相比,一步涂层法具有操作过程简单、材料消耗少、制备薄膜质量好等优点,可用于大批量制备高效率量子点太阳电池。     

FULLSPECTRUM: a new PV wave making more efficient use of the solar spectrum

The project FULLSPECTRUM — an Integrated Project (IP) in the terminology of the European Commission — pursues a better exploitation of the FULL solar SPECTRUM by (1) further developing concepts already scientifically proven but not yet developed and (2) by trying to prove new ones in the search for a breakthrough in photovoltaic (PV) technology. More specific objectives are the development of: (a) III–V multijunction cells (MJC), (b) solar thermo-photovoltaic (TPV) converters, (c) intermediate band (IB) materials and cells (IBC), (d) molecular-based concepts (MBC) for full PV utilisation of the solar spectrum and (e) manufacturing technologies (MFG) for novel concepts including assembling. MJC technology towards 40% efficiency will be developed using lower cost substrates and high light concentration (up or above 1000 suns). TPV is a concept with a theoretically high efficiency limit because the entire energy of all the photons is used in the heating process and because the non-used photons can be fed back to the emitter, therefore helping in keeping it hot. In the IBC approach, sub-bandgap photons are exploited by means of an IB. Specific IB materials will be sought by direct synthesis suggested by material-band calculations and using nanotechnology in quantum dot (QD) IBCs. In the development of the MBC, topics such as the development of two-photon dye cells and the development of a static global (direct and diffuse) light concentrator by means of luminescent multicolour dyes and QDs, with the radiation confined by photonic crystals, will be particularly addressed. MFG include optoelectronic assembling techniques and coupling of light to cells with new-optic miniconcentrators.     

Novel radioluminescent nuclear battery: Spectral regulation of perovskite quantum dots

CsPbBr₃ and CsPbBr_1.5I_1.5 perovskite quantum dots (QDs) are synthesized by hot‐injection with PPO (2,5‐diphenyloxazole) as a fluorescent material for radioluminescent nuclear battery. The results reveal that the fluorescence of the QD/PPO system consists of radioluminescence (4.79%‐5.35%) and photoluminescence (nearly 95%). The addition of QDs leads to more excellent optical and electrical properties of radioluminescent nuclear battery. The peak position of the radioluminescence spectra of QD/PPO can be regulated by controlling the components of QDs. This strategy is suitable for obtaining a satisfactory spectral matching factor for different photovoltaic devices to obtain outstanding output performance. Moreover, good selection of QD/PPO as a fluorescent material can significantly improve the overall output performance of the radioluminescent nuclear battery. The linear relationship between optical and electrical properties was presented. Perovskite QDs exhibit excellent application prospects for the (α, β, γ, and X‐ray sources) radioluminescent nuclear battery and X‐ray imaging technology.     

Efficiency enhancement for bulk-heterojunction hybrid solar cells based on acid treated CdSe quantum dots and low bandgap polymer PCPDTBT

We report on the efficiency enhancement for bulk-heterojunction hybrid solar cells based on hexanoic acid treated trioctylphosphine/oleic acid-capped CdSe quantum dots (QDs) and low bandgap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) compared to devices based on poly(3-hexylthiophene) (P3HT). Photovoltaic devices with optimized polymer:QD weight ratio, photoactive film thickness, thermal annealing treatment, and cathode materials exhibited a power conversion efficiency of 2.7% after spectral mismatch correction, which is the highest reported value for spherical CdSe QD based photovoltaic devices. The efficiency enhancement is attributed to the surface treatment of the QDs together with the use of the low bandgap polymer PCPDTBT leading to an increased short-circuit current density due to additional light absorption between 650 and 850 nm. Our results suggest that the hexanoic acid treatment is generally applicable to various ligand-capped CdSe and confirm that low bandgap polymers with adequate HOMO and LUMO levels are promising to be incorporated into hybrid solar cells for further device performance improvement.     

Progress on hot carrier cells

Hot Carrier cells aim to collect hot carriers resulting from photon absorption before they thermalise. This requires slowed carrier cooling in an absorber material and collection through narrow energy selective contacts. Previous work has proved the concept of these energy selective contacts using resonant tunnelling in Si quantum dot (QD) double-barrier structures. Further experimental work on electrical and optical excitations of these structures is shown. Modelling work has demonstrated the possibility of slowing the rate of carrier cooling by modifying the phonon dispersion in QD superlattices. Developments in this modelling which indicate the critical importance of the interface are also presented.     

Near-infrared CdSexTe1-x@CdS “giant” quantum dots for efficient photoelectrochemical hydrogen generation

Colloidal quantum dots (QDs) have attracted a lot of attention due to their unique optoelectronic properties. They have been widely used as building block materials for solar technologies such as solar cell, and photoelectrochemical (PEC) water splitting. Hydrogen generation by using QDs as photocatalysts has emerged a promising application in PEC devices. However, it is still very challenging to obtain high-efficiency PEC devices due to the limited absorption wavelength of QDs and the existence of surface traps which prohibit the efficient charge transfer. In this work, we synthesized ternary CdSe_xTe_1-x/CdS (CdSeTe/CdS) “giant” QDs to extend the light absorption to near infrared, matched well with Sun's spectrum. The as-synthesized CdSeTe/CdS “giant” QDs exhibit quasi-type II band alignment as confirmed by its long lifetime and red-shifted emission peak compared with bare CdSeTe QDs. The wide absorption range of “giant” core/shell QDs and their long lifetime can improve the efficient absorption of Sun's spectrum and charge transfer. As a proof-of-concept, a PEC device using QDs sensitized TiO₂ mesoporous thin film as a photoanode was used for hydrogen production. The corresponding photocurrent density was increased to 3.0 mA/cm² with the introduction of CdS shell, which is 1.5 times higher than the PEC device using CdSeTe QDs. This study indicates that ternary or polynary alloyed core/shell QDs can be used as promising optoelectronic materials for applications of PEC devices.     

A numerical study of Bi-periodic binary diffraction gratings for solar cell applications

In this paper, a numerical study is made of simple bi-periodic binary diffraction gratings for solar cell applications. The gratings consist of hexagonal arrays of elliptical towers and wells etched directly into the solar cell substrate. The gratings are applied to two distinct solar cell technologies: a quantum dot intermediate band solar cell (QD-IBSC) and a crystalline silicon solar cell (SSC). In each case, the expected photocurrent increase due to the presence of the grating is calculated assuming AM1.5D illumination. For each technology, the grating period, well/tower depth and well/tower radii are optimised to maximise the photocurrent. The optimum parameters are presented. Results are presented for QD-IBSCs with a range of quantum dot layers and for SSCs with a range of thicknesses. For the QD-IBSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with less than 70 quantum dot layers. In a QD-IBSC with 50 quantum dot layers equipped with the optimum grating, the weak intermediate band to conduction band transition absorbs roughly half the photons in the corresponding sub-range of the AM1.5D spectrum. For the SSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with thicknesses of 10 μm or greater. A 20 μm thick SSC equipped with the optimised grating leads to an absorption enhancement above that of a 200 μm thick SSC equipped with a planar back reflector.     

Thermal effect on bound exciton in CdTe/Cd1−xZnxTe cylindrical quantum dots

The temperature dependence of the exciton ground state, lowest and binding energies in cylindrical quantum dot (QD) are theoretically investigated using a variational procedure within the effective mass approximation. The interaction between the charge carriers (electron and hole) and longitudinal optical (LO) phonon modes is taken into account. The excitonic confinement is described by a finite depth potential well. Specific applications of these results are given for CdTe QDs embedded in a CdTe/Cd_1-xZn_xTe matrix. The total and lowest energies depend strongly on the temperature, up to a critical value of temperature T (around 40 °K), increasing with it. In the small dot sizes region, both energies decrease rapidly with increasing dot radius and approach the quantum well limit energies for large radius. In addition, an enhancement of the Zn fraction in the barrier material increases both the total and binding energies of the exciton. It is found that the exciton binding energy is significantly reduced with increasing temperature and its effect is more important on the ground state energy than on the binding energy. Moreover the exciton is stable even at room temperature. The LO-phonon contribution to the exciton binding energy is important and depends on the dot size, the Zn fraction and the temperature.     

Solar collector operating temperatures for maximum coefficient of performance of an absorption refrigeration system

An attempt has been made to determine the optimum operating temperatures of a linear solar concentrator with a tubular receiver for maximum coefficient of performance of an absorption refrigeration system. The effects of absorber temperature, emissivity of absorber and wind loss coefficient on the heat loss factor has been taken into account in obtaining the optimum system performance.     

CdSe quantum dot-single wall carbon nanotube complexes for polymeric solar cells

The development of lightweight, flexible polymeric solar cells which utilize nanostructured materials has been investigated. Incorporation of quantum dots (QDs) and single wall carbon nanotubes (SWNTs) into a poly(3-octylthiophene)-(P3OT) composite, has been shown to facilitate exciton dissociation and carrier transport in a properly structured device. Optimization towards an ideal electron acceptor for polymeric solar cells that exhibits high electron affinity and high electrical conductivity has been proposed in the form of QD-SWNT complexes. Specifically, the synthesis of CdSe-aminoethanethiol-SWNT complexes has been performed, with confirmation by microscopy (SEM, TEM, and AFM) and spectroscopy (FT-IR and optical absorption). Polymer composites containing these complexes in P3OT have been used to fabricate solar cells which show limited efficiency due to recombination and surface effects, but an open-circuit voltage (V_OC) of 0.75 V. However, evaluation of the optical absorption spectra for these nanomaterial-polymeric composites has shown a marked enhancement in the ability to capture the available irradiance of the air mass zero (AM0) spectrum.